Conventionally, for identifying the position of a terminal device (hereinafter, “terminal”) including a function for performing wireless communication, a method using RSSI (Receive Signal Strength Indication) is used. Conventionally, for example, first, the RSSI from plural fixed stations (for example, at N points) located around the terminal are measured, and the RSSI values are converted into distances, so that the distances between the respective fixed stations and the terminal are acquired. Subsequently, the position where the terminal is located is determined by combining the distances from the plural fixed stations.
Furthermore, conventionally, there is the following technology estimating the position of a mobile station by triangulation using distances obtained with RSSI from plural base stations. Specifically, the position of the mobile station is estimated using RSSI of two or more positions, the transmission power and the antenna gain are known, and the attenuation tilt of the distance to the propagation path (attenuation rate) that is not known is obtained by RSSI measurement. Furthermore, conventionally, there is a technology of creating an RSSI map by measuring RSSI of one or more (plural) receiving points using the base station, and calculating the environmental impact (including the distance to attenuation amount) of a propagation path attenuation model. Furthermore, conventionally, there is a technology of obtaining the distance between a wireless terminal to a base station, using base station information including transmission power and an antenna gain, RSSI of the wireless terminal received from the base station, and a predetermined estimate equation of propagation loss such as the “Okumura-Hata Model”. Furthermore, conventionally, there is a technology of obtaining the relationship between the electric field strength (RSSI) and the distance by sequentially changing the distance to the mobile station from the fixed station (for example, by bringing the mobile station closer by 5 m), and obtaining the distance by measuring the electric field strength from three or more fixed stations, to detect the position of a mobile station (see, for example, patent documents 1 through 4).    Patent document 1: Japanese Laid-Open Patent Publication No. 2010-160158    Patent document 2: Japanese Laid-Open Patent Publication No. 2007-68163    Patent document 3: Japanese Laid-Open Patent Publication No. 2010-232920    Patent document 4: Japanese Laid-Open Patent Publication No. 11-218571
However, as described above, when positioning (identifying the position of) a positioning target object such as a terminal by using RSSI of radio waves from plural fixed stations (for example, at N points), the transmission power of a fixed station whose position is registered in advance may change due to the environment. Therefore, conventionally, the accurate distance to the terminal may not be obtained, and errors may arise in the positioning results.
The RSSI to a terminal that is located at a communication distance d from a fixed station is obtained by the following formula (1).RSSI=Ptx+Gtx−Loss—d+Grx  (1)
In the above formula (1), Ptx indicates the transmission power at the fixed station (access point), Gtx indicates the transmission antenna gain at the fixed station, Loss_d indicates the loss of space in the communication distance d, and Grx indicates the reception antenna gain at the terminal. Furthermore, the transmission power Ptx and the transmission antenna gain Gtx are parameters unique to a fixed station, the loss of space Loss_d is a parameter determined by a distance, and the reception antenna gain Grx is a parameter unique to a terminal.
The terminal acquires the RSSI and substitutes, into the above formula (1), the acquired RSSI, the transmission power Ptx, the transmission antenna gain Gtx, and the reception antenna gain Grx, and calculates the loss of space Loss_d. Furthermore, the terminal acquires the distance to the fixed station based on the loss of space Loss_d obtained as a result of the calculation. Furthermore, the terminal acquires the above difference from three different fixed stations to identify the position of itself.
The parameters unique to a fixed station described above may change due to the direction (angle) of the fixed station and the installation method of the fixed station (for example, the distance from a wall). Similarly, the parameter that is determined by a distance may change due to the environment of the propagation space (for example, when it is raining or when there is a radio disturbance due to an obstacle). For example, the loss of space Loss_d in the above formula (1) that is a parameter that is determined by a distance may be obtained by the following formula (2).loss of space Loss—d=10 log(4πd/λ)n  (2)
In the above formula (2), λ is the wavelength of the radio wave in the space, and n is the attenuation rate in the propagation path. The attenuation rate in formula (2) is square (n=2) in a free space; however, this value (the attenuation rate) increases in an environment that is affected by reflection.
Furthermore, the above parameter unique to a terminal may change according to the direction (angle) of the terminal and the degree of adhesion (how the terminal is being held) between the terminal and a human body.
The transmission power Ptx that is one of the parameters unique to the fixed station is typically determined to be a uniform value by a wireless standard. Furthermore, the wavelength λ that is one of the parameters determined by a distance is typically determined to be a uniform value by a wireless standard, and may vary somewhat due to the difference in the channel (ch), although such a difference is negligible. Furthermore, the reception antenna gain Grx that is one of the parameters unique to a terminal may be a value that is set in advance in each terminal.
Therefore, in the formula (1), if the distance d from the fixed station is determined, there are only two unknown variables, i.e., the transmission antenna gain Gtx and the attenuation rate of loss of space n.
However, in the above patent documents 1 through 4, there are no processes for correcting the two unknown variables described above, and therefore it is not possible to efficiently acquire correction information for performing positioning appropriately.